924Board.org

[tumai]

hi!
I'm a new member from France, owner of a 1986 924S LY7Y. I've got this car since last October, and I'm still puzzled by some vibrations.

They come from the engine, since they are present even with the clutch released (pedal pressed). They can be felt at approx. 2100 and 4200 rpm (6300 I didn't try).

At first I thought of a belt problem, but all the belts have been changed, without any modification a far as the vibrations are concerned. I'm thinking about changing the engine mounts, but this may only mask the problem without solving it.

Any of you had similar vibs??

best regards
Tumi

[John Brown]

Bad engine mounts can 'cause' vibration. So a repair by replacement of the mounts is proper and not just a masking.

Another thing to be aware of is the proper indexing (timing) of the balance shafts when the belts were replaced.
_________________
John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[tumai]

Thanks for the answer, especially concerning the engine mounts.

As for the balance shafts, as hinted in my first post, I don't consider them a possible cause anymore, since the intensity and eigen-frequency of the vibs weren't modified by the change of belt. It would be very surprising that the same error (exactly) could occur twice.

Any possibility of faulty ignition ?

I'm more and more considering changing the mounts.

Tumi

[gohim]

If the Balance Shaft Belt is not indexed correctly, you are going to get vibration.

If you replaced the Balance Shaft Belt exactly the way it was, and it was incorrectly indexed, you would still have vibration before and after replacement.

[Lizard]

I would suggest replacing the mounts as they are probably dead anyways, but it doesnt take long to pull the front covers off turn to TDC and check that the balance belt is indeed lined upcorrectly.
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3 928s,

[genikz88]

I just replaced both my engine mounts and I still have the same vibration in my exhaust that I had with my old and worn out original mounts. It comes on around 2500 rpm and smooths out around 4000 rpm.

You might focus on the balance shaft belt first, but inspect your mounts for sure. The balance shaft belt is much easier to get to than the mounts!
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Chris
'88 924S

[9xx]

I've had engine vibration caused by motor mounts, twice. Engine vibrated and caused rattle only on idle but when I pushed the throtte, it went away.

And it stayed smooth even in high revs. It only rattled under 1200rpm. In both times one of the mounts was totally collapsed.
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Mikko

All gone: 931 '82 Alpine White, original option "220" G31 with LSD + 3 x 944

[Browar]

Last post is correct. Bad engine mounts usually can be identified by vibrations at idle until 1200 rpm. Maybe later they are present too but a idle engine gets a lot of vibrations.
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Browar
eS
924 S `87

[tumai]

I don 't have much vibrations at low rpm, just at 2100 and 4200. I've been told that vibrations at low rpm are caused mainly by the balance shaft ... (by people who removed purely the belt for use in races)

In my case, since it is not the balance shaft belt, and if the mounts don't cause vibs at high rpm, what else???

tumi

[ronined]

[9xx]

[ronined]

9xx,

Thanks for the info and the link. I didn't think to check out Clark's Garage.
I have a question on step #11 which states,

"Support the engine under the oil pan using a floor jack and a piece of plywood (as a jacking pad). The piece of plywood should be large enough to evenly distribute the weight of the engine over a good size area. I normally use a 12" x 12" piece of plywood."

This plywood should probably be at least 1/2 inch thick I would think, maybe 3/4 inch?

Ed
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'87 924S

[Rasta Monsta]

I would be hesitant to jack on the oil pan, although it seems a lot a people do it. If there is a place to attach a come-along, and lift the engine that way, your oil pan gasket will be safer!
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[9xx]

[tumai]

OK it WAS the balance shaft. the lower one as a matter of facts, it was positioned with a 180° error ...

Now, the car is almost unrecognisable!

thanks everybody

tumi